The present invention relates to a method for holographic conversion of one wave being emitted from a first object, into another wave which is identical to the wave another but geometrically similar object would emit at a wavelength which may be different from the wavelength of the former wave. The objects may be selfradiating or consist of scattering elements, and the converted wave may be of another physical nature than the original wave.
In holography a wave front to be recorded is brought to interfere physically or synthetically with a reference wave front and the interference pattern is recorded in a hologram. Subsequently the recorded wave is reconstructed physically either in its original or in a converted form. If the actual reconstruction wave is equal to the reference wave, the reconstructed wave is identical to the recorded wave. If the reconstruction wave is different from the reference wave, the reconstructed wave gets converted in relation to the recorded wave. Until now it has been a requirement that the reference wave front and the reconstruction wave front be similarly shaped in the hologram.
Conversion of waves as mentioned above has been known previously in connection with linear scaling of holograms. By this known type of wave conversion, however, one cannot obtain a scaled reconstructed wave which is shaped similarly to the recorded wave, unless the hologram, the object and the wavelength all are scaled linearly by the same factor. With a recording wavelength .lambda..sub.1 and a different reconstruction wavelength .lambda..sub.2 a change of the linear hologram dimensions by a factor 1/M will result in a transverse (parallel to the hologram) positional translation of points in the object by a factor M, and a longitudinal positional translation of points by a factor M.sup.2 .lambda..sub.1 /.lambda..sub.2. Consequently, the reconstructed object gets distorted throughout its depth unless the factor M is chosen equal to .lambda..sub.2 /.lambda..sub.1. For practical reasons the proper choice of the factor M may be made only in cases in which the wavelength differences are minor. In all other cases the reconstructed object usually gets so small that it is necessary to enlarge it optically in order to obtain a satisfactory parallax-effect. By such magnification, however, the depth distortion is regenerated.
The scaling problem has been treated thoroughly in the literature, but has not been solved. In the descriptions found in the patent literature concerning acoustic holography and in particular seismic holography, directions for linear scaling of the wave front as recorded in the hologram plane have been given by way of a number of examples. By such scaling a compromise is made with regard to the wavelength ratio in order to obtain suitable hologram dimensions. Thus the reconstructed object is suitably scaled, but inevitably gets distorted. In connection with scaling of acoustic holograms it has been stated in the literature that the difference between the recording wavelength and the reconstruction wavelength excludes the possibility of obtaining a realistic three-dimensional reconstruction of the object.